A typical radio access network (RAN) includes one or more base stations, each radiating to define a respective coverage area, such as a cell and cell sector, in which user equipment devices (UEs) such as cell phones, tablet computers, tracking devices, embedded wireless modules, and other wirelessly equipped communication devices, can operate. Further, each base station of the RAN may then be coupled or communicatively linked with network infrastructure such as a switch or gateway that provides connectivity with one or more transport networks, such as the public switched telephone network (PSTN) and/or the Internet for instance. With this arrangement, a UE within coverage of the RAN may thus engage in air interface communication with a base station and thereby communicate via the base station with various remote network entities or with other UEs served by the RAN.
In general, a RAN may operate in accordance with a particular air interface protocol or radio access technology, with communications from a base station to UEs defining a downlink or forward link and communications from the UEs to the base station defining an uplink or reverse link. Examples of existing air interface protocols include, without limitation, Orthogonal Frequency Division Multiple Access (OFDMA) (e.g., Long Term Evolution (LTE) or Wireless Interoperability for Microwave Access (WiMAX)), Code Division Multiple Access (CDMA) (e.g., 1×RTT and 1×EV-DO), Global System for Mobile Communication (GSM), WI-FI, and BLUETOOTH. Each protocol may define its own procedures for registration of UEs, initiation of communications, handover between coverage areas, and functions related to air interface communication.
In accordance with the air interface protocol, each of the coverage areas of such a RAN may operate on one or more carrier frequencies and may define a number of air interface channels for carrying information between the base station and UEs. By way of example, each coverage area may define a pilot channel, reference channel or other resource on which the base station may broadcast a pilot signal, reference signal, or the like that UEs may detect as an indication of coverage and may measure to evaluate coverage strength. Further, each coverage area may define a downlink control channel for carrying system information, page messages, and other control signaling from the base station to UEs, and an uplink control channel for carrying service requests and other control signaling from UEs to the base station, and each coverage area may define downlink and uplink traffic channels or the like for carrying bearer traffic between the base station and UEs.
When a UE initially enters into coverage of a RAN (e.g., powers on or moves into coverage of the RAN), the UE may detect a reference signal and read system information broadcast from a base station and may engage in a process to register itself to be served by the base station and generally by the RAN. For instance, the UE may transmit an attach message on an uplink control channel to the base station, and the base station and/or supporting infrastructure may then responsively authenticate and authorize the UE for service, establish a record indicating where in the RAN the UE is operating, establish local profile or context records for the UE, and provide an attach accept message to the UE. Thereafter, the UE may then be served by the RAN in an idle mode or a connected/active mode. In the idle mode, the UE may monitor a downlink control channel to detect page messages and other information regarding incoming communications and may similarly transmit uplink control signaling to initiate communications or for other reasons. And in the connected/active mode, the UE may have particular traffic channel resources assigned by the RAN, which the UE may use to engage in communication of bearer traffic and the like.
When a UE is served in a particular base station coverage area, the UE may also regularly monitor the reference signal strength in that coverage area and in other coverage areas of the RAN, in an effort to ensure that the UE operates in the best (e.g., strongest) coverage area. If the UE detects threshold weak coverage from its serving coverage area and sufficiently strong coverage from another coverage area, the UE may then engage in a handover process by which the UE transitions to be served by the other coverage area. In the idle mode, the UE may do this autonomously and might re-register in the new coverage area. Whereas, in the connected/active mode, the UE may regularly report signal strengths to its serving base station when certain thresholds are met, and the base station and/or supporting infrastructure may work to hand the UE over to another coverage area.
Some wireless communication systems may encompass multiple separate but interconnected RANs, such as a first RAN that provides high speed data communications and a second RAN (i.e., a “fallback” RAN) that provides traditional telephony service, each RAN operating in accordance with a different air interface protocol. Such a hybrid system may be configured to support an inter-RAN fallback process in which the first RAN passes various signaling, such as pre-registration signaling and call-setup signaling, between the fallback RAN and UEs served by the first RAN in order to facilitate transition of UEs from being served by the first RAN to being served with voice calls by the fallback RAN. For instance, a hybrid system that includes an LTE RAN for data communications and a circuit-switched RAN, such as a CDMA RAN (or GSM RAN or the like), for legacy telephone service may support an example inter-RAN fallback process referred to as “circuit-switched fallback” (CSFB). With such an arrangement, a UE may be programmed to scan for and attach with the LTE RAN by default and to then register with the CDMA RAN by engaging in pre-registration signaling through the LTE RAN. In particular, after the UE attaches with an LTE base station, the UE may then transmit to the LTE base station a CDMA pre-registration request message, and that message may pass through the LTE RAN to the CDMA RAN, leading to pre-registration of the UE with the CDMA RAN. Thereafter, when the UE has a voice call to place or the CDMA RAN has a voice call to connect to the UE, call-setup signaling may similarly pass between the UE and the CDMA RAN via the LTE RAN, at which point the UE may then conveniently switch over to be served by a CDMA base station and to engage in the voice call.
In a hybrid system, a UE may be arranged to support both service provided by the first RAN and service provided by the fallback RAN, and further might be arranged to engage in signaling with the fallback RAN via the first RAN to facilitate transition of the UE from being served by the first RAN to being served with voice calls by the fallback RAN. For instance, such a UE may include both a first radio for being served by the first RAN and a second radio for being served by the fallback RAN, and may also include program logic that allows the UE to engage in the inter-RAN fallback process described above. A UE that is arranged in this manner may be referred to herein as a UE that supports the inter-RAN fallback process. In the context of an LTE/CDMA hybrid system, for instance, such a UE may be referred to herein as a “CSFB-capable” UE. Further, when a base station of the first RAN is arranged to pass signaling between the UE and the fallback network that will serve the UE with a voice call as described above, such a base station may be referred to as a base station that supports the inter-RAN fallback process or a base station that supports CSFB.